You can only add/subtract those that have matching square roots. So 4√5 and -2√5 go together. 2√7 and -3√7 go together
***Important!!! Only add/subtract the numbers outside the square root together. The number inside the square root stays the same
(4√5 +(-2√5) ) + (2√7 + (-3√7) )
2√5 + (-1√7)
2√5 - √7
I assume that is what you wanted right? Let me know and I hope this helped!